Fixes "function" test against regular expressions

In some browser engines (especially older versions of Webkit), this
test fails when the argument is a RegExp.

See:
http://stackoverflow.com/questions/5054352/why-use-typeof-for-identifyin
g-a-function

Fix tested and works with Adobe AIR.

lib/less/parser.js

@@ -224,7 +224,7 @@ less.Parser = function Parser(env) {
     }
 
     function expect(arg, msg) {
-        var result = (typeof arg === "function") ? arg.call(parsers) : $(arg);
+        var result = (Object.prototype.toString.call(arg) === '[object Function]') ? arg.call(parsers) : $(arg);
         if (result) {
             return result;
         }